Fault locator for testing a complex optical circuit

ABSTRACT

A fiber optic tester ( 10 ) broadly comprises a testing unit ( 16 ) to take measurements across two test points ( 27 ), a processing unit ( 18 ) to locate faults by analyzing the measurements, a switching unit ( 20 ) that can connect termination points ( 13 ) of a electrical circuit ( 12 ) to the test points ( 27 ) in a sequence controlled by the processing unit ( 18 ), and a fiber unit ( 22 ) to test a optical circuit ( 14 ). The tester ( 10 ) may also include an electrical harness ( 24 ) or an optical harness to connect the electrical circuit ( 12 ) to the switching unit ( 20 ) or the optical circuit ( 14 ) to the fiber unit ( 22 ). The processing unit ( 18 ) is preferably programed with interconnection information of the circuits ( 12, 14 ) and internal characteristics of the tester ( 10 ). Using the interconnection information and the internal characteristics, the processing unit ( 18 ) may accurately detect faults within the circuits ( 12, 14 ).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fiber optic testers. More particularly,the present invention provides a fiber optic tester that canautomatically and accurately test electrical and optical circuits in amanufacturing environment.

2. Description of Prior Art

Fiber optics and optical circuits are increasingly used in production ofaircraft, vehicles, and equipment. Such optical circuits must be tested,to ensure high quality and reliability. Automated testing of opticalcircuits is becoming necessary as optical circuits become more complexmaking manual testing laborious and subject to human error.

For example, handheld fiber testing units are widely used to testoptical circuits. However, handheld fiber testing units cannot test morethan one fiber strand at a time and require technicians to repeatedlyperform several complicated steps flawlessly in order to obtain accurateresults. For example, a technician is typically required to individuallyconnect both ends of each fiber strand to a handheld fiber testing unitbefore individually testing each fiber strand.

Handheld fiber testing units also typically require significant trainingand are not well suited to operation by unspecialized labor. Forexample, the steps required to correctly connect ends of fiber strandsto a handheld fiber testing unit may be complex. Any error in makingsuch connects drastically effects test results. Thus, training and skillare extremely important to insure accurate test results with handheldfiber testing units.

Furthermore, such units are typically very sensitive and cannotwithstand rough treatment commonly found in manufacturing environments.For example, hand held fiber testing units are typically destroyed, ifdropped or otherwise jarred. Thus, handheld fiber testing units arebetter suited to use with few fiber strands by highly trained andspecialized labor in laboratory environments.

Finally, most fiber testing units are not able to accurately testrelatively short fiber strands, such as 20–30 feet. Most fiber strandsare rated for attenuation over long distances, such as kilometers,because attenuation over short distances is relatively minimal. In fact,short fiber stands typically have less than one decibel of attenuation.Thus, errors made in testing short fiber strands are typically moresignificant than those made in testing long fiber strands.

Additionally, electrical circuits are also commonly used in productionof aircraft, vehicles, and equipment along with optical circuits.Currently, technicians must use two different devices to test opticaland electrical circuits. For example, technicians may be required to usea handheld fiber testing unit to test optical circuits and anotherdevice to test electrical circuits. Such devices commonly operate quitedifferently, thereby requiring technicians to be familiar with twocompletely different testing procedures.

Accordingly, there is a need for an improved fiber optic tester thatovercomes the limitations of the prior art.

SUMMARY OF THE INVENTION

The present invention overcomes the above-identified problems andprovides a distinct advance in the art of testers. More particularly,the present invention provides a fiber optic tester that canautomatically and accurately test both an electrical and an opticalcircuit. The tester broadly comprises a testing unit to takemeasurements across two test points, a processing unit to locate faultsby analyzing the measurements taken by the testing unit, a switchingunit that can connect termination points of the electrical circuit tothe test points in a sequence controlled by the processing unit, and afiber unit to test the optical circuit. The tester may also include anelectrical harness to connect the electrical circuit to the switchingunit and/or an optical harness to connect the fiber unit to the opticalcircuit. Alternatively, the switching unit and/or the fiber unit may bedesigned to mate directly with the circuits.

ELECTRICAL TESTING

The electrical circuit may include several hundred termination points,with each termination point preferably wired to at least one othertermination point according to design requirements and application ofthe electrical circuit. For example, the electrical circuit may includetermination points, such as A1, A2, B1, B2, C1, C2, and X1.Additionally, the electrical circuit may include a plurality of terminalblocks, such as TB1, TB2, and TB3. The electrical circuit may alsoinclude other common electrical elements, such as, resistors,capacitors, inductors, lamps, switches, diodes, and fuses. Furthermore,the electrical circuit may comprise any combination of circuit boardtraces and/or bundles of various wiring types, such as coaxial wire,twisted pair, shielded wire, and individual conductors.

The electrical circuit may be of the type used in aircraft, othervehicles, backplanes, appliances, black-boxes, and/or other complicatedequipment. For example, the electrical circuit may be designed toprovide interconnections for a Boeing model 767 commercial jet airliner.Alternatively, the electrical circuit may be designed to provideinterconnections for a light-rail train, an automobile, printed circuitboard, or a super computer. In any case, the electrical circuit mayinclude many wire runs or interconnections each of differing length,wire size, and/or type. Additionally, the termination points mayterminate in individual connectors, be grouped into collectiveconnectors, or use a combination of connectors for the terminationpoints.

The testing unit preferably comprises a first and second test point,such as T1 and T2, a precision current source connected across the testpoints, and a precision voltage sensor also connected across the testpoints. The testing unit preferably takes a differential voltagemeasurement using the voltage sensor to sense voltage across the testpoints. The testing unit also preferably takes a resistance measurementusing the current source to apply current through the test points andthe voltage sensor to sense voltage across the test points, which isdirectly proportional to the resistance measurement. Dividing thevoltage across the test points by the current through the test pointsyields the resistance measurement, according to Ohm's Law.

Similarly, the testing unit preferably takes a capacitance measurementusing the current source to apply current through the test points andthe voltage sensor to sense voltage across the test points. However, intaking the capacitance measurement, the testing unit preferably timeshow long it takes to reach a specified voltage across the test points,which is directly proportional to the capacitance measurement. In otherwords, the longer it takes to reach the specified voltage, the higherthe capacitance measurement.

The testing unit and the switching unit preferably operate together andunder control of the processing unit. As discussed above, the testingunit preferably includes two test points, and yet is preferably able totake the resistance measurement between each pair of termination pointsand the capacitance measurement across each termination point and areference point, which is preferably an unused termination point, suchas X1. Thus, the switching unit facilitates connecting the test pointsto each of the termination points.

The switching unit preferably comprises a pair of single pole singlethrow relays, such as R1, R2, R3, R4, R5, and R6, connected to eachtermination point. A first relay is preferably connected to T1 and asecond relay is preferably connected to T2. The relays are preferablycontrolled by an addressing unit that individually energizes the relaysaccording to digital address words received from the processing unitover an address bus. The address words direct which termination pointshould be connected to which test point, and thus instruct theaddressing unit which relays should be energized.

As discussed above, the processing unit is preferably programed with theelectrical circuit's interconnection information, thereby allowing theprocessing unit to know which combinations of termination points shouldresult in high and low resistance measurements. Thus, the processingunit preferably compares each resistance measurement to theinterconnection information to identify which termination points are inconflict and in what manner the termination points conflict, therebydetermining what faults exist in the electrical circuit. However, it istypically insufficient for the processing unit to simply determine whichtermination points are in conflict, since this doesn't effectivelyinform a service technician where he or she should begin in order toactually locate and fix the faults. Without more information, thetechnician must spend many hours trying to locate and fix the faults.

In order to help the technician locate and fix the faults, theprocessing unit is preferably able to use the measurements to determinewhere each fault is located in the electrical circuit. For example, ifTB1 is shorted to X1, then the testing unit would measure relatively lowresistance between any combination of A1, B1, and X1. While theprocessing unit may expect the resistance measurement to be relativelylow between A1 and B1, the resistance measurement between A1 and X1 isexpected to be relatively high. Additionally, the resistance measurementbetween B1 and X1 is also expected to be relatively high. Since theresistance measurements involving X1 are relatively low, then theprocessing unit determines that there must be a short-circuit fault(SCF) to X1 somewhere between A1 and B1.

The processing unit next identifies where, between A1 and B1, the SCF islocated by comparing the corresponding resistance measurements. As anexample, suppose the resistance measurement between A1 and B1 is foundto be approximately 0.2461 Ohms, the resistance measurement between A1and X1 is found to be approximately 0.2166 Ohms, and the resistancemeasurement between B1 and X1 is found to be approximately 0.1085 Ohms.Since the resistance measurement between B1 and X1 is the smallestvalue, the SCF must be closer to B1 than to A1.

The processing unit can be even more precise by adding the resistancemeasurement between A1 and X1 to the resistance measurement between B1and X1 which yields a sum of approximately 0.3251. The sum, in thiscase, inherently includes the resistance between X1 and the SCF measuredtwice and the resistance measurement between A1 and B1. It can be seenthat, the sum is approximately 0.0790 Ohms greater than the resistancemeasurement between A1 and B1. Therefore, the resistance between X1 andthe SCF is approximately 0.0395 Ohms.

In order to locate the SCF with respect to A1 and B1, the 0.0395resistance can be subtracted from the resistance measurement between A1and X1 and the resistance measurement between B1 and X1. In this case,the resistance between A1 and the SCF is approximately 0.1771 Ohms andthe resistance between B1 and the SCF is approximately 0.0690 Ohms. Theprocessing unit then calculates a resistance ratio (RR) of approximately72%, in this case, indicating that the SCF is located approximately 72%of a conductor length (CL) between A1 and B1. Thus, in this case, theprocessing unit preferably indicates that the SCF is locatedapproximately 72% of the CL from A1.

To be even more precise, the interconnection information programmed intothe processing unit preferably includes details as to electricalcharacteristics and CLs as well as wire sizes and types used throughoutthe electrical circuit. For example, suppose the CL from A1 to TB1 issupposed to be approximately 151 inches and the CL from B1 to TB1 issupposed to be approximately 59 inches. Applying the RR to a totalconductor length (TCL) between A1 and B1 of approximately 210 inchesyields that the SCF must be approximately 151 inches from A1 orsubstantially adjacent TB1. Therefore, the processing unit preferablyindicates that the SCF is located substantially adjacent TB1. In thiscase, it is anticipated that the technician's first actions will centeraround examining TB1. Thus, by identifying the location of the fault andnearby items, such as TB1, the tester of the present invention guidesthe technician directly to the location of the fault.

Additionally, the interconnection information may comprise positionalinformation relating to physical paths of the electrical circuit. Usingthe positional information and the determined location of the fault, theprocessing unit can determine an actual position of the fault. Forexample, assuming that the electrical circuit is installed in a vehicle,the processing unit can inform the technician where the fault may bephysically found with reference to the vehicle. More specifically, theprocessing unit may inform the technician that the fault is locatedbehind a specific access panel, within a specific section of conduit, orin a specific junction box, etc.

It should be apparent that finding faults comprises detecting andreporting which termination points are in conflict. Locating faultscarries this much further in actually determining and reporting wherefaults are located along the electrical circuit, drastically cuttingtime that the technician must spend looking for the fault. Finally,positioning faults further advances the art by determining and reportingwhere faults may be found with reference to their surroundings.

In preferred embodiments, the tester is capable of locating orpositioning faults within the electrical circuit. However, inalternative embodiments, the tester may not actually locate faults andmay be limited to finding faults. Additionally, in still otherembodiments, the tester may include very limited, if any, electricaltesting capability. Although, it is anticipated that the tester will bemost useful if able to fully test both the electrical circuit and theoptical circuit.

The processing unit locates open-circuit faults by performingcalculations on the capacitance measurement. For example, if there is anopen-circuit fault (OCF) between TB2 and TB3, then the testing unitwould measure relatively high resistance between either A2 or C2 andeither B2 or C1. However, programmed with the interconnectioninformation, the processing unit expects relatively low resistancebetween any combination of A2, B2, C1, and C2. Additionally, in thiscase, the resistance measurement between A2 and C2 is also relativelylow, as expected. Furthermore, in this case, the resistance measurementbetween B2 and C1 is relatively low, as expected. Therefore, theprocessing unit determines that A2 and C2 are both connected to TB2 andB2 and C1 are both connected to TB3, as expected. Thus, the processingunit determines that the OCF is located somewhere between TB2 and TB3.

In order to more accurately locate the OCF, the processing unit nextcompares the corresponding capacitance measurements with respect to thereference point, such as X1. As an example, suppose the capacitancemeasurement for A2 is approximately 1247 picoFarads (pF), thecapacitance measurement for B2 is approximately 308 pF, the capacitancemeasurement for C1 is approximately 323 pF, and the capacitancemeasurement for C2 is approximately 1236 pF. It should be noted that thecapacitance measurement for A2 is expected to be substantially identicalto the capacitance measurement for C2, since A2 is connected to C2through the electrical circuit. Likewise, the capacitance measurementfor B2 is expected to be substantially identical to the capacitancemeasurement for C1. In the above example, there are slight variationswhich may occur for any number of reasons, such as repeatability of thevoltage sensor and other internal influences. In any case, more accurateresults may be obtained by averaging the values expected to beidentical. Thus, in this case, an average for the TB2 side of the OCF isapproximately 1242 pF and an average for the TB3 side of the OCF isapproximately 316 pF. Then, the processing unit calculates a capacitanceratio (CR) of approximately 80%, indicating that the OCF is locatedapproximately 80% of the TCL between A2 to C2 and B2 to C1.

Additionally, using the interconnection information, the processing unitcan calculate the TCL or amount of conductive path on each side of theOCF. For example, suppose the CL from A2 to TB2 is approximately 196inches, suppose the CL from C2 to TB2 is approximately 84 inches,suppose the CL from B2 to TB3 is approximately 38 inches, suppose the CLfrom C1 to TB3 is approximately 36 inches, and suppose the CL from TB2to TB3 is approximately 42 inches. In this case, the TCL isapproximately 396 inches. Applying the CR, the OCF is located atapproximately 317 inches along the TCL between A2 to C2 and B2 to C1.Since the CL from A2 to C2 is approximately 280 inches, the OCF must beapproximately 37 inches from TB2 and approximately 5 inches from TB3. Inthis case, the processing unit preferably indicates that the OCF isapproximately 37 inches from TB2 and approximately 5 inches from TB3,leading the technician to first examine TB3.

In the above examples, the RR and the CR that are calculated by theprocessing unit may or may not precisely locate the fault. For example,either the RR or the CR may be off by as much as 10%. With simpleelectrical circuits, a 10% error may be acceptable and still adequatelyguide the technician to the fault. However, with more complex or verylong electrical circuits, the 10% error may not be acceptable requiringthe technician to examine several terminal blocks.

The majority of the error experienced by the tester may be due tointernal influences caused by manufacturing tolerances of internalcomponents and other factors. Thus, the tester can be even more accurateby compensating for such internal influences. There are at least twointernal influence reduction methods (IIRM) that the tester of thepresent invention may use in compensating for internal influences. Afirst IIRM involves simply reversing the test points used in calculatingthe RR and the CR during a first iteration, such as that describedabove. For example, if the switching unit was directed to connect A1 toT1 and B1 to T2, then the processing unit performs a second iterationdirecting the switching unit to connect A1 to T2 and B1 to T1. Uponcompletion of the second iteration, the processing unit preferablyaverages the RRs and the CRs developed during the first and seconditerations. In this manner, the processing unit can minimize at leastsome of the internal influences of the tester.

A second IIRM involves the processing unit subtracting values stored ina tare log, which will be discussed in more detail below, from eachmeasurement taken by the testing unit. As will become apparent, thiseffectively subtracts the internal influences of the tester from thecalculations used to develop the RR and the CR. Additionally, theprocessing unit may develop an extremely accurate RR and CR by using acombination of both the first IIRM and the second IIRM.

The tare log is developed during calibration of the tester andeffectively stores internal characteristics of the tester. Theseinternal characteristics, such as an internal resistance and an internalcapacitance, contribute to the internal influences described above. Forexample, a total ratio of the resistance measurement between A1 and X1to the resistance measurement between B1 and X1 is approximately 67%. Itcan be seen that, the total ratio differs from the RR by 5% of the TCL,which is directly related to the influences of the resistance between X1and the SCF. The internal characteristics of the tester may effect theRR and the CR in a similar manner. Thus, the processing unit preferablysubtracts the internal resistance and the internal capacitance of thetester as described above as was done for the resistance between X1 andthe SCF.

The internal resistance is preferably determined by first shorting theswitching unit where the electrical circuit or the electrical harnesswould normally be connected. Then, the processing unit directs theswitching unit to cycle through every possible combination oftermination points, while the testing unit takes the resistancemeasurements. The processing unit logs each resistance measurement inthe tare log for each combination of termination points, therebyidentifying the internal resistance of the fault locator without theelectrical harness.

Additionally, the electrical harness may be shorted where the electricalcircuit would normally be connected. Then, the processing unit againdirects the switching unit to cycle through every possible combinationof termination points, while the testing unit takes the resistancemeasurements. The processing unit again logs each resistance measurementin the tare log for each combination of termination points, therebyidentifying the internal resistance of the fault locator with theelectrical harness.

Alternatively, the internal resistance of the electrical harness may becalculated. For example, individual internal resistances for eachconductor of the electrical harness can be calculated based on length,cross-section, and material for each conductor. Each individual internalresistance may be stored in the tare long and added to the internalresistance for the corresponding combination of termination points.

Similarly, the internal capacitance is preferably determined with theelectrical harness connected to the switching unit but disconnected fromthe electrical circuit. The processing unit then directs the switchingunit to cycle through each termination point, while the testing unittakes the capacitance measurements. The processing unit logs eachcapacitance measurement in the tare log for each termination point,thereby identifying the internal capacitance of the tester with theelectrical harness. Additionally, or where the electrical harness is notused, the switching unit itself may be left disconnected from both theelectrical harness and the electrical circuit. In this case, theprocessing unit logs each capacitance measurement in the tare log foreach combination of termination points, thereby identifying the internalcapacitance of the tester without the electrical harness. It isimportant to note that where the electrical harness is used, the tarelog preferably contains the internal characteristics of the tester bothwith and without the electrical harness.

The processing unit can not only improve the accuracy of the RR and theCR, as described above, but may even locate faults within the testeritself, using the internal characteristics stored in the tare log. Forexample, if the internal resistance of the tester with the electricalharness is greater than the resistance measurement calculated duringtesting of the electrical circuit, then the SCF must be locatedsomewhere within the tester or the electrical harness. The processingunit may also determine more precisely where the SCF is located usingthe internal resistance associated of the tester without the electricalharness. For example, if the resistance measurement calculated duringtesting of the electrical circuit is between the internal resistance ofthe tester with and without the electrical harness, then the SCF must belocated within the electrical harness. Alternatively, if the resistancemeasurement calculated during testing of the electrical circuit is lessthan the internal resistance of the tester without the electricalharness, then the SCF must be located within the tester itself. It isimportant to note that the processing unit may locate both SCFs and OCFswithin the tester itself and/or the electrical harness using theprocedures discussed above.

It can be seen that the tester uses percentages and ratios in locatingthe faults. This allows the processing unit to disregard environmentalconcerns, such as temperature effects on the electrical circuit, sincethe concerns will effect the tester, the electrical circuit, and theelectrical harness, thereby biasing all measurements in a similarmanner. Thus, using ratios effectively negates any errors environmentalconcerns may otherwise induce.

FIBER OPTICAL TESTING

The fiber unit preferably allows the tester to perform a lightattenuation measurement on each of a plurality of fiber strands withinthe optical circuit. The attenuation measurement is similar to theresistance measurement described above. Each fiber strand preferablyincludes two termination points, such as F1, F2, F3, and F4, with one ofthose termination points on either end of the optical circuit.

Thus, the interconnection information associated with the opticalcircuit is expected to be simpler than that associated with theelectrical circuit. For example, the interconnection informationassociated with the optical circuit is expected to include informationas to which pairs of termination points are expected to beinterconnected. The interconnection information may also includeinformation, such as fiber sizes, lengths, and types.

The fiber unit is preferably designed to individually test each fiberstrand and preferably comprises a plurality of fiber optic light ports,with at least one light port for each termination point of the opticalcircuit. Each light port preferably comprises a light transmitter totransmit light into one of the fiber strands through an optical coupler,a light monitor to monitor the light from the light transmitter throughthe coupler, and a light receiver to receive light from the lighttransmitter through one of the fiber strands. The light transmitterpreferably comprises a light emitting diode (LED) powered by a constantcurrent source.

The coupler preferably divides the light transmitted by the lighttransmitter into two equal halves, such as a fiber power (PF) and amonitor power (PM). The coupler preferably directs PF toward one of thefiber strands of the optical circuit and PM to the light monitor. Thecoupler also preferably divides the light received from one of the fiberstrands into two equal haves, such as a receiver power (PR) and a wastepower (PW). It should be noted the PW is preferably directed toward thetransmitter and has virtually no significance.

Thus, the coupler allows the tester to transmit and receive light witheach light port through each fiber strand. Using the coupler, the testermay test each fiber strand in both directions, without any form of fiberswitching or reversing of connectors. In fact, all fiber components arepreferably solid-state resulting in a durable and rugged tester that canbe moved around in a production environment and still be relied upon tomake accurate, stable, and repeatable attenuation measurements.

The light monitor preferably produces a monitor voltage (VM), which ispreferably logarithmically proportional to PM, or the light receivedfrom the light transmitter through the coupler. The light receiver ispreferably similar to the light monitor and preferably produces areceiver voltage (VR), which is preferably logarithmically proportionalto PR, or the light received from the fiber strand through the coupler.

As discussed above, two light ports are preferably used to test eachfiber strand. For example, the light transmitter of a first light portis preferably used to transmit the light into one of the fiber strands,with the light monitor of the first light port monitoring that lighttransmitter. At the same time, the light receiver of a second light portis preferably used to receive light from that fiber strand. Thus, thelight monitor detects how much light is transmitted into that fiberstrand while the light receiver detects how much light is received fromthat fiber strand.

The light monitor and the light receiver are preferably connected to amonitor differential amplifier and a receiver differential amplifier,respectively. The receiver amplifier preferably amplifies VRapproximately twice as much as the monitor amplifier amplifies VM. Thedifferential amplifiers preferably amplify VM and VR with respect to areference ground bus. The differential amplifiers are preferablyconnected to an analog bus switch that provides functionality similar tothe switching unit described above. The analog bus switch preferablyconnects either the monitor differential amplifier or the receiverdifferential amplifier to one of the test points of the testing unitaccording to instructions received by the processing unit. For example,with T1 connected to one of the monitor differential amplifiers and T2connected to one of the receiver differential amplifiers through theanalog bus switch, the voltage sensor can take the voltage measurementbetween VM and VR for one of the fiber strands.

It should be obvious that PR is substantially half of PF minus anyattenuation loss in that fiber strand. Since PM substantially equals PF,PR is expected to be substantially half of PM minus the attenuation lossin that fiber strand. As discussed above, VM is preferably proportionalto PM and VR is preferably proportional to PR. Additionally, thereceiver amplifier preferably amplifies VR such that VR is expected tobe substantially equal to VM minus the attenuation loss in that fiberstrand. Thus, the voltage measurement between VM and VR is indicative ofthe attenuation loss in that fiber strand.

The processing unit preferably indicates the attenuation measurement foreach fiber strand. Additionally, if any attenuation measurement exceedsa programmed attenuation limit, the processing unit also preferablyindicates that the associated fiber strand has failed. For example, thetester may take attenuation measurements of a specific fiber strand infirst and second directions. If the attenuation limit is approximately 4dB and the attenuation measurement is approximately 3.8 dB in the firstdirection and approximately 4.1 dB in the second direction, then theprocessing unit preferably indicates both attenuation measurements andthat the specific fiber strand has failed.

As discussed above, the tare log is developed during calibration of thetester and stores internal characteristics of the tester. The internalcharacteristics may include attenuation characteristics of the fiberunit. Thus, the processing unit can compensate for internalcharacteristics of the fiber unit, as well as other internal influencesof the tester, as described above.

The processing unit of the tester is preferably controlled and monitoredthrough a user interface program. The program preferably allows thetechnician to view and print a report detailing the measurements takenfrom the circuits. Additionally, the program may allow the technician toprovide the interconnection information. Furthermore, the program mayallow the technician to specify the manner in which the processing unittests the circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is described in detailbelow with reference to the attached drawing figures, wherein:

FIG. 1 is a block diagram of a tester constructed in accordance with apreferred embodiment of the present invention connected to a electricalcircuit and a optical circuit;

FIG. 2 is a schematic of the electrical circuit showing a simpleshort-circuit fault and a simple open-circuit fault;

FIG. 3 is a schematic of a testing unit of the tester;

FIG. 4 is a schematic of a switching unit of the tester;

FIG. 5 is a detail schematic of a relay of the switching unit;

FIG. 6 is a schematic of the electrical circuit showing a more complexshort-circuit fault;

FIG. 7 is a schematic of the optical circuit;

FIG. 8 is a schematic fiber unit of the tester; and

FIG. 9 is a detail schematic of a light port of the fiber unit.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, the preferred fiber optic tester 10 constructed inaccordance with a preferred embodiment of the present invention isillustrated connected to an electrical circuit 12 having a plurality oftermination points 13 and an optical circuit 14 also having a pluralityof termination points 15. The tester 10 broadly comprises a testing unit16 to take measurements, a processing unit 18 to analyze themeasurements taken by the testing unit 16, a switching unit 20 that canconnect the electrical circuit 12 to the testing unit 16 in a sequencecontrolled by the processing unit 18, and a fiber unit 22 that can beused to test the optical circuit 14. In a preferred embodiment, thetester 10 also includes an electrical harness 24 designed to connect theelectrical circuit 12 to the switching unit 22 and/or an optical harnessto connect the fiber unit 22 to the optical circuit 14. Alternatively,the switching unit 20 and/or the fiber unit 22 may be designed to matedirectly with the circuits 12, 14.

ELECTRICAL TESTING

The electrical circuit 12 may include several hundred termination points13, with each termination point 13 wired to at least one othertermination point 13 according to design requirements and application ofthe electrical circuit 12. For example and referring also to FIG. 2, theelectrical circuit 12 may include termination points, such as A1, A2,B1, B2, C1, C2, and X1 interconnected as shown. It is important to notethat while FIG. 2 only shows seven termination points 13, in theinterest of simplicity and clarity, it is to be understood that theelectrical circuit 12 is expected to be much more complex. Additionally,the electrical circuit 12 may include a plurality of terminal bocks 26,such as TB1, TB2, and TB3. The electrical circuit 12 may also includeother common electrical elements, such as resistors, capacitors,inductors, lamps, switches, diodes, and fuses. Furthermore, theelectrical circuit 12 may comprise any combination of circuit boardtraces and/or bundles of various wiring types, such as coaxial wire,twisted pair, shielded wire, and individual conductors.

The electrical circuit 12 may be of the type used in aircraft, othervehicles, backplanes, appliances, black-boxes, and/or other complicatedequipment. For example, the electrical circuit 12 may be designed toprovide interconnections for a Boeing model 767 commercial jet airliner.Alternatively, the electrical circuit 12 may be designed to provideinterconnections for a light-rail train, an automobile, printed circuitboard, or a super computer. In any case, the electrical circuit 12 mayinclude many wire runs each of differing length, wire size, and/or type.Additionally, the termination points 13 may terminate in individualconnectors, be grouped into collective connectors, or use a combinationof connectors.

As will be discussed in greater detail below, the tester 10 ispreferably programmed with interconnection information which is uniqueto the electrical circuit 12 and/or the optical circuit 14. However, itis anticipated that the tester 10 will be used to test many differentcircuits. For example, a specific aircraft may include more than oneunique circuit. While the tester 10 must be programmed for each uniquecircuit, the tester 10 may be used to test each unique circuit in thespecific aircraft or may be used to test unique circuits from severalaircraft.

For example, it is commonplace in modern manufacturing to break down acomplex task into several smaller tasks and perform each of the smallertasks at a different place or time in a manufacturing process. Thus, themanufacturing process may incorporate several testers 10, with eachtester 10 programmed to test only one of several unique circuits that gointo the specific aircraft. In this manner, each tester 10 may beprogrammed once and then used to test many unique circuits each destinedto go into a different specific aircraft. Alternatively, themanufacturing process may utilize only one tester 10 and reconfigurethat tester 10 to test each unique circuit that goes into the specificaircraft.

Referring also to FIG. 3, the testing unit 16 preferably comprises afirst and second test point 27, such as T1 and T2, a precision currentsource 28 connected across the test points 27, and a precision voltagesensor 30 also connected across the test points 27. The testing unit 16preferably takes a differential voltage measurement using the voltagesensor 30 to sense voltage across the test points 27. Additionally, thetesting unit 16 preferably takes a resistance measurement using thecurrent source 28 to apply current through the test points 27 and thevoltage sensor 30 to sense voltage across the test points 27, which isdirectly proportional to the resistance measurement. Dividing thevoltage across the test points 27 by the current through the test points27 yields the resistance measurement, according to Ohm's Law.Alternatively, the testing unit 16 may use any other commonly usedtechnique to take the resistance measurement.

Similarly, the testing unit 16 preferably takes a capacitancemeasurement using the current source 28 to apply current through thetest points 27 and the voltage sensor 30 to sense voltage across thetest points 27. However, in taking the capacitance measurement, thetesting unit 16 preferably times how long it takes to reach a specifiedvoltage across the test points 27, which is directly proportional to thecapacitance measurement. In other words, the longer it takes to reachthe specified voltage, the higher the capacitance measurement.Alternatively, the testing unit 16 may use any other commonly usedtechnique to take the capacitance measurement.

The testing unit 16 preferably communicates the measurements to theprocessing unit 18 over a measurement bus as digital data words. In analternative embodiment, the testing unit 16 may provide a voltage analogsignal proportional to the voltage measurement, a resistance analogsignal proportional to the resistance measurement, and a capacitanceanalog signal proportional to the capacitance measurement. In this casethe testing unit 16 relies on an analog to digital converter in theprocessing unit 18 to convert the analog signals into the digital datawords. In still another alternative embodiment, the testing unit 16 maynot include any calculating capability. As such, the testing unit 16 mayapply current through the test points 27 according to instructions fromthe processing unit 18 and provide the processing unit 18 with thevoltage across the test points 27, in either digital or analog form. Inthis manner, the processing unit 18 itself may calculate themeasurements. Thus, the testing unit 16 may actually take themeasurements or may merely facilitate or assist the processing unit 18in taking the measurements.

The testing unit 16 and the switching unit 20 may be designedspecifically for the electrical circuit 12. For example, the testingunit 16 and the switching unit 20 may be designed to test high voltageelectrical circuits. As such, the testing unit 16 and the switching unit20 may be designed to operate at up to 2000 volts. Alternatively, thetesting unit 16 and the switching unit 20 may be designed to test highcurrent electrical circuits. As such, the testing unit 16 and theswitching unit 20 may be designed to operate at up to 100 amps.

However, the testing unit 16 and the switching unit 20 may be designedto test both high voltage and high current electrical circuits.Furthermore, the tester 10 may be capable of testing both high voltageand/or high current electrical circuits as well as low voltage and/orlow current electrical circuits. In this case, the testing unit 16 andthe switching unit 20 may be modular and replaceable in order to testdifferent electrical circuits. Alternatively, the testing unit 16 andthe switching unit 20 may be designed to test both high voltage and/orhigh current electrical circuits as well as low voltage and/or lowcurrent electrical circuits without requiring replacement.

The testing unit 16 and the switching unit 20 preferably operatetogether and under control of the processing unit 18. As discussedabove, the testing unit 16 preferably includes two test points 27, andyet is able to take the resistance measurement between each pair oftermination points 13 and the capacitance measurement across eachtermination point 13 and a reference point, which is preferably anunused termination point, such as X1. Thus, the switching unit 20facilitates connecting the test points 27 to each of the terminationpoints 13 of the electrical circuit 12.

Referring also to FIG. 4, the switching unit 20 preferably comprises apair of single pole single throw relays 32, such as R1, R2, R3, R4, R5,and R6, connected to each termination point 13. Referring also to FIG.5, a first relay 34 is preferably connected to T1 and a second relay 36is preferably connected to T2. The relays 32 are preferably controlledby an addressing unit 40 that individually energizes the relays 32according to digital address words received from the processing unit 18over an address bus. The address words direct which termination point 13should be connected to which test point 27, and thus instruct theaddressing unit 40 which relays 32 should be energized.

For example, the processing unit 18 may direct the addressing unit 40 ofthe switching unit 20 to actuate selected relays 32 such that A1 isconnected to T1 and B1 is connected to T2. In this case, the addressingunit 40 preferably energizes R1 such that A1 is connected to T1 throughthe first relay 34. Additionally, the addressing unit 40 preferablyenergizes R6 such that B1 is connected to T2 through the second relay36. The addressing unit 40 also preferably de-energizes the remainingrelays 32, such that the corresponding termination points 13 aredisconnected from the test points 27. With the switching unit 20configured in this manner, the testing unit 16 can take the resistancemeasurement between A1 and B1.

Alternatively, the processing unit 18 may direct the switching unit 20to actuate selected relays 32 such that A1 is connected to T1 and X1 isconnected to T2. In this case, the addressing unit 40 preferablyenergizes R1 such that A1 is connected to T1 through the first relay 34.Additionally, the addressing unit 40 preferably energizes R14 such thatX1 is connected to T2 through the second relay 36. The addressing unit40 also preferably de-energizes the remaining relays 32, such that thecorresponding termination points 13 are disconnected from the testpoints 27. With the switching unit 20 configured in this manner, thetesting unit 16 can take the resistance measurement between A1 and X1.

Furthermore, the processing unit 18 may direct the switching unit 20 toactuate selected relays 32 such that X1 is connected to T1 and B1 isconnected to T2. In this case, the addressing unit 40 preferablyenergizes R13 such that X1 is connected to T1 through the first relay34. Additionally, the addressing unit 40 preferably energizes R6 suchthat B1 is connected to T2 through the second relay 36. The addressingunit 40 also preferably de-energizes the remaining relays 32, such thatthe corresponding termination points 13 are disconnected from the testpoints 27. With the switching unit 20 configured in this manner, thetesting unit 16 can take the capacitance measurement across B1 and X1.

As discussed above, the processing unit 18 is preferably programed withthe electrical circuit's 12 interconnection information, therebyallowing the processing unit 20 to know which combinations oftermination points 13 should result in high and low resistancemeasurements. Thus, the processing unit 18 compares each resistancemeasurement to the interconnection information to identify whichtermination points 13 are in conflict and in what manner the terminationpoints 13 conflict, thereby determining what faults exist in theelectrical circuit 12. However, it is typically insufficient for theprocessing unit 18 to simply determine which termination points 13 arein conflict, since this doesn't effectively inform a service technicianwhere he or she should begin in order to actually locate and fix thefaults. Without more information, the technician may be required tospend many hours trying to locate and fix the faults.

In order to help the technician locate and fix the faults, theprocessing unit 18 is able to use the measurements to determine whereeach fault is located in the electrical circuit 12. For example, if TB1is shorted to X1, as shown by the dashed line in FIG. 2, then thetesting unit 16 would measure relatively low resistance between anycombination of A1, B1, and X1. While the processing unit 18 may expectthe resistance measurement to be relatively low between A1 and B1, theresistance measurement between A1 and X1 is expected to be relativelyhigh. Additionally, the resistance measurement between B1 and X1 is alsoexpected to be relatively high. Since the resistance measurementsinvolving X1 are relatively low, then the processing unit 18 determinesthat there must be a short-circuit fault (SCF) to X1 somewhere betweenA1 and B1.

The processing unit 18 next identifies where, between A1 and B1, the SCFis located by comparing the corresponding resistance measurements. As anexample, suppose the resistance measurement between A1 and B1 is foundto be approximately 0.2461 Ohms, the resistance measurement between A1and X1 is found to be approximately 0.2166 Ohms, and the resistancemeasurement between B1 and X1 is found to be approximately 0.1085 Ohms.Since the resistance measurement between B1 and X1 is the smallestvalue, the SCF must be closer to B1 than to A1.

The processing unit 18 can be even more precise by adding the resistancemeasurement between A1 and X1 to the resistance measurement between B1and X1 which yields a sum of approximately 0.3251. The sum, in thiscase, inherently includes the resistance between X1 and the SCF measuredtwice and the resistance measurement between A1 and B1. It can be seenthat, the sum is approximately 0.0790 Ohms greater than the resistancemeasurement between A1 and B1. Therefore, the resistance between X1 andthe SCF is approximately 0.0395 Ohms.

In order to locate the SCF with respect to A1 and B1, the 0.0395resistance can be subtracted from the resistance measurement between A1and X1 and the resistance measurement between B1 and X1. In this case,the resistance between A1 and the SCF is approximately 0.1771 Ohms andthe resistance between B1 and the SCF is approximately 0.0690 Ohms. Theprocessing unit 18 then calculates a resistance ratio (RR) ofapproximately 72%, in this case, indicating that the SCF is locatedapproximately 72% of a conductor length (CL) between A1 and B1. As shownin FIG. 2, A1 is interconnected with B1 through TB1 and TB1 is roughlythree times as far from A1 as from B1. Thus, in this case, theprocessing unit 18 preferably indicates that the SCF is locatedapproximately 72% of the CL and near TB1.

To be even more precise, the interconnection information programmed intothe processing unit 18 preferably includes details as to electricalcharacteristics and CLs as well as wire sizes and types used throughoutthe electrical circuit 12. For example, suppose the CL from A1 to TB1 issupposed to be approximately 151 inches and the CL from B1 to TB1 issupposed to be approximately 59 inches. Applying the RR to a totalconductor length (TCL) between A1 and B1 of approximately 210 inchesyields that the SCF must be approximately 151 inches from A1 orsubstantially adjacent TB1. Therefore, the processing unit 18 preferablyindicates that the SCF is located substantially adjacent TB1. Theprocessing unit 18 may also indicate the CL to the SCF from A1 and/orB1. In this case, it is anticipated that the technician's first actionswill center around examining TB1. Thus, by identifying the location ofthe fault and nearby items, such as TB1, the tester 10 of the presentinvention guides the technician directly to the location of the fault.

Additionally, the interconnection information may comprise positionalinformation relating to physical paths of the electrical circuit 12.Using the positional information and the determined location of thefault, the processing unit 18 can determine an actual position of thefault. For example, assuming that the electrical circuit 12 is installedin a vehicle, the processing unit 18 can inform the technician where thefault may be physically found with reference to the vehicle. Morespecifically, the processing unit 18 may inform the technician that thefault is located behind a specific access panel, within a specificsection of conduit, or in a specific junction box, etc.

It should be apparent that finding faults comprises detecting andreporting which termination points 13 are in conflict. Locating faultscarries this much further in actually determining and reporting wherefaults are located along the electrical circuit 12, drastically cuttingtime that the technician must spend looking for the fault. Finally,positioning faults further advances the art by determining and reportingwhere faults may be found with reference to their surroundings.

In preferred embodiments, the tester 10 is capable of locating orpositioning faults within the electrical circuit 12. However, inalternative embodiments, the tester 10 may not actually locate faultsand may be limited to finding faults. Additionally, in still otherembodiments, the tester 10 may include very limited, if any, electricaltesting capability. Although, it is anticipated that the tester 10 willbe most useful if able to fully test both the electrical circuit 12 andthe optical circuit 14.

The processing unit 18 locates open-circuit faults by performingcalculations on the capacitance measurement. For example, if there is anopen-circuit fault (OCF) between TB2 and TB3, as shown by the diagonallines in FIG. 2, then the testing unit 16 would measure relatively highresistance between either A2 or C2 and either B2 or C1. However,programmed with the interconnection information, the processing unit 18expects relatively low resistance between any combination of A2, B2, C1,and C2. Additionally, in this case, the resistance measurement betweenA2 and C2 is also relatively low, as expected. Furthermore, in thiscase, the resistance measurement between B2 and C1 is relatively low, asexpected. Therefore, the processing unit 18 determines that A2 and C2are both connected to TB2 and B2 and C1 are both connected to TB3, asexpected. Thus, the processing unit 18 determines that the OCF islocated somewhere between TB2 and TB3.

In order to more accurately locate the OCF, the processing unit 18 nextcompares the corresponding capacitance measurements with respect to thereference point, such as X1. As an example, suppose the capacitancemeasurement for A2 is approximately 1247 picoFarads (pF), thecapacitance measurement for B2 is approximately 308 pF, the capacitancemeasurement for C1 is approximately 323 pF, and the capacitancemeasurement for C2 is approximately 1236 pF. It should be noted that thecapacitance measurement for A2 is expected to be substantially identicalto the capacitance measurement for C2, since A2 is connected to C2through the electrical circuit 12. Likewise, the capacitance measurementfor B2 is expected to be substantially identical to the capacitancemeasurement for C1. In the above example, there are slight variationswhich may occur for any number of reasons, such as repeatability of thevoltage sensor 30 and other internal influences. In any case, moreaccurate results may be obtained by averaging the values expected to beidentical. Thus, in this case, an average for the TB2 side of the OCF isapproximately 1242 pF and an average for the TB3 side of the OCF isapproximately 316 pF. Then, the processing unit 18 calculates acapacitance ratio (CR) of approximately 80%, indicating that the OCF islocated approximately 80% of the TCL between A2 to C2 and B2 to C1.

As discussed above, the interconnection information programmed into theprocessing unit 18 includes details as to CLs as well as wire sizes andtypes used throughout the electrical circuit 12. Therefore, theprocessing unit 18 can calculate the TCL or amount of conductive path oneach side of the OCF. For example, suppose the CL from A2 to TB2 isapproximately 196 inches, suppose the CL from C2 to TB2 is approximately84 inches, suppose the CL from B2 to TB3 is approximately 38 inches,suppose the CL from C1 to TB3 is approximately 36 inches, and supposethe CL from TB2 to TB3 is approximately 42 inches. In this case, the TCLis approximately 396 inches. Applying the CR, the OCF is located atapproximately 317 inches along the TCL between A2 to C2 and B2 to C1.Since the CL from A2 to C2 is approximately 280 inches, the OCF must beapproximately 37 inches from TB2 and approximately 5 inches from TB3. Inthis case, the processing unit 18 preferably indicates that the OCF isapproximately 37 inches from TB2 and approximately 5 inches from TB3,leading the technician to first examine TB3.

It is important to note that using only the CL, as described above,assumes that relevant conductors are of sufficiently identical types.Several factors, such as shielding, insulation, wire size, and material,can influence the above described calculations. For example, a largerwire size between A2 and TB2 than between B2 and TB3 may bias thecapacitance measurement associated with A2 higher than the capacitancemeasurement associated with B2. However, the processing unit 18 maycompensate for these and other factors using the interconnectioninformation.

In the above examples, the RR and the CR that are calculated by theprocessing unit 18 may or may not precisely locate the fault. Forexample, either the RR or the CR may be off by as much as 10%. Withsimple electrical circuits, such as that shown in FIG. 2, a 10% errormay be acceptable and still adequately guide the technician to thefault. However, with more complex or very long electrical circuits, the10% error may not be acceptable requiring the technician to examineseveral terminal blocks 26.

The majority of the error experienced by the tester 10 may be due tointernal influences caused by manufacturing tolerances of internalcomponents and other factors. Thus, the tester 10 can be even moreaccurate by compensating for such internal influences. There are atleast two internal influence reduction methods (IIRM) that the tester 10of the present invention may use in compensating for internalinfluences. A first IIRM involves simply reversing the test points 27used in calculating the RR and the CR during a first iteration, such asthat described above. For example, if the switching unit 20 was directedto connect A1 to T1 and B1 to T2, then the processing unit 18 performs asecond iteration directing the switching unit 20 to connect A1 to T2 andB1 to T1. Upon completion of the second iteration, the processing unit18 preferably averages the RRs and the CRs developed during the firstand second iterations. In this manner, the processing unit 18 canminimize at least some of the internal influences of the tester 10.

A second IIRM involves the processing unit 18 subtracting values storedin a tare log, which will be discussed in more detail below, from eachmeasurement taken by the testing unit 16. As will become apparent, thiseffectively subtracts the internal influences of the tester 10 from thecalculations used to develop the RR and the CR. Additionally, theprocessing unit 18 may develop an extremely accurate RR and CR by usinga combination of both the first IIRM and the second IIRM.

The tare log is developed during calibration of the tester 10 andeffectively stores internal characteristics of the tester 10. Theseinternal characteristics, such as an internal resistance and an internalcapacitance, contribute to the internal influences described above. Forexample, a total ratio of the resistance measurement between A1 and X1to the resistance measurement between B1 and X1 is approximately 67%. Itcan be seen that, the total ratio differs from the RR by 5% of the TCL,which is directly related to the influences of the resistance between X1and the SCF. The internal characteristics of the tester 10 may effectthe RR and the CR in a similar manner. Thus, the processing unit 18preferably subtracts the internal resistance and the internalcapacitance of the tester 10 as described above as was done for theresistance between X1 and the SCF.

The internal resistance is preferably determined by first shorting theswitching unit 20 where the electrical circuit 12 or the harness 24would normally be connected. Then, the processing unit 18 directs theswitching unit 20 to cycle through every possible combination oftermination points 13, while the testing unit 16 takes the resistancemeasurements. The processing unit 18 logs each resistance measurement inthe tare log for each combination of termination points 13, therebyidentifying the internal resistance of the tester 10 without the harness24.

Additionally, the harness 24 may be connected to the switching unit 20and shorted at the circuit end where the electrical circuit 12 wouldnormally be connected. Then, the processing unit 18 again directs theswitching unit 20 to cycle through every possible combination oftermination points 14, while the testing unit 16 takes the resistancemeasurements. The processing unit 18 again logs each resistancemeasurement in the tare log for each combination of termination points13, thereby identifying the internal resistance of the tester 10 withthe harness 24.

Alternatively, the internal resistance of the harness 24 may becalculated. For example, individual internal resistances for eachconductor of the harness 24 can be calculated based on length,cross-section, and material for each conductor. Each individual internalresistance may be stored in the tare log and added to the internalresistance for the corresponding combination of termination points 13.

Similarly, the internal capacitance is preferably determined with theelectrical harness 24 connected to the switching unit 20 butdisconnected from the electrical circuit 12. The processing unit 18 thendirects the switching unit 20 to cycle through every possiblecombination of termination points 13, while the testing unit 16 takesthe capacitance measurements. The processing unit 18 logs eachcapacitance measurement in the tare log for each combination oftermination points 13, thereby identifying the internal capacitance ofthe tester 10 with the electrical harness 24. Additionally, or where theelectrical harness 24 is not used, the switching unit 20 itself may beleft disconnected from both the electrical harness 24 and the electricalcircuit 12. In this case, the processing unit 18 logs each capacitancemeasurement in the tare log for each combination of termination points13, thereby identifying the internal capacitance of the tester 10without the electrical harness 24. It is important to note that wherethe electrical harness 24 is used, the tare log preferably contains theinternal characteristics of the tester 10 both with and without theelectrical harness 24.

The processing unit 18 can not only improve the accuracy of the RR andthe CR, as described above, but may even locate faults within the tester10 itself, using the internal characteristics stored in the tare log.For example, if the internal resistance of the tester 10 with theelectrical harness 24 is greater than the resistance measurementcalculated during testing of the electrical circuit 12, then the SCFmust be located somewhere within the tester 10 or the electrical harness24. The processing unit 18 may also determine more precisely where theSCF is located using the internal resistance associated of the tester 10without the electrical harness 24. For example, if the resistancemeasurement calculated during testing of the electrical circuit 12 isbetween the internal resistance of the tester 10 with and without theelectrical harness 24, then the SCF must be located within theelectrical harness 24 and may be referred to as a harness fault.Alternatively, if the resistance measurement calculated during testingof the electrical circuit 12 is less than the internal resistance of thetester 10 without the electrical harness 24, then the SCF must belocated within the tester 10 itself and may be referred to as a testerfault. It is important to note that the processing unit 18 may locateboth SCFs and OCFs within tester 10 itself and/or the electrical harness24. As such, the harness faults and the tester faults may be either SCFsor OCFs.

It can be seen that the tester 10 uses percentages and ratios inlocating the faults. This allows the processing unit 18 to disregardenvironmental concerns, such as temperature effects on the wire bundle12, since the concerns will effect the tester 10, the electrical circuit12, and the electrical harness 24, thereby biasing all measurements in asimilar manner. Thus, using ratios effectively negates any errorsenvironmental concerns may otherwise induce.

The interconnection information also preferably includes details as towhat electrical elements are included in the electrical circuit 12,where those elements are located, and what characteristics thoseelements posses. Thus, the processing unit 18 can bias the calculationsused to develop the RR and the CR by the characteristics of theelements. Similarly, as discussed above, the processing unit 18 can biasthe calculations used to develop the RR and the CR by the different wiresizes that may exist in the electrical circuit 12. For example, if thewire between A2 and TB2 is larger that the wire between TB2 and C2, thenthe resistance between A2 and TB2 may actually be lower than theresistance between TB2 and C2. Thus, the processing unit 18 may bias theRR calculations, such as through dividing the resistance measurement bya cross-sectional area of the wire or multiplying by a gauge size.

As discussed above, while FIG. 2 shows a relatively simple example ofthe electrical circuit 12, the tester 10 is expected to be used withmuch more complex examples of the electrical circuit 12. Referring alsoto FIG. 6, in an effort to illustrate a more complex example of theelectrical circuit 12, suppose TB1 is shorted to somewhere between TB2and TB3, as shown by the dashed line. In this case, the testing unit 16would measure relatively low resistance between any combination of thetermination points 13 and manually locating the SCF may be an extremelydifficult matter for the technician. However, the tester 10 simplyutilizes the procedures described above, allowing the processing unit 18to accurately locate virtually any fault regardless of the complexity ofthe electrical circuit 12. For example, Table 1 shows exemplaryresistance measurements that may be taken by the testing unit 16, inexamining the electrical circuit 12 of FIG. 6. Notice that some of theCLs are unknown, since those termination points 13 are not supposed tobe connected.

TABLE 1 Resistance Measurements Associated with FIG. 6 Points CL(inches) Resistance (Ohms) A1 to A2 — 0.4418 A1 to B1 210 0.2461 A1 toB2 — 0.2590 A1 to C1 — 0.2566 A1 to C2 — 0.3106 A2 to B1 — 0.3340 A2 toB2 276 0.3234 A2 to C1 274 0.3211 A2 to C2 280 0.3281 B1 to B2 — 0.1512B1 to C1 — 0.1488 B1 to C2 — 0.2027 B2 to C1  74 0.0867 B2 to C2 1640.1922 C1 to C2 162 0.1898

However, using the procedures described above, the tester 10 can takeand utilize measurements for each possible combination of terminationpoints 13. For example, adding the resistance measurement between A1 andB2 to the resistance measurement between B1 and B2 yields a sum ofapproximately 0.4102. The sum, in this case, inherently includes theresistance between B2 and the SCF measured twice and the resistancemeasurement between A1 and B1. It can be seen that, the sum isapproximately 0.1641 Ohms greater than the resistance measurementbetween A1 and B1. Therefore, the resistance between B2 and the SCF isapproximately 0.0820 Ohms. In this case it is important to note that the0.0820 Ohm resistance also includes any resistance due to the shortitself. Regardless, it follows that the resistance between A1 and theSCF is approximately 0.1770 Ohms and the resistance between B1 and theSCF is approximately 0.0692 Ohms. In this case, as in the first example,the processing unit 18 calculates the resistance ratio (RR) to beapproximately 72%, indicating that the SCF is located approximately 72%of the CL between A1 and B1 or substantially adjacent TB1. This gives usa first conflicting location of the SCF which can be verified comparingA1 and B1 with C1 or C2, in the same manner as was done with B2.

In order to find a second conflicting location of the SCF, we mustexamine the electrical circuit 12 from an opposing perspective. Forexample, adding the resistance measurement between A1 and A2 to theresistance measurement between A1 and B2 yields a sum of approximately0.7008. The sum, in this case, inherently includes the resistancebetween A1 and the SCF measured twice and the resistance measurementbetween A2 and B2. It can be seen that, the sum is approximately 0.3774Ohms greater than the resistance measurement between A2 and B2.Therefore, the resistance between A1 and the SCF is approximately 0.1887Ohms. In this case it is important to note that the 0.1887 Ohmresistance also includes any resistance due to the short itself.Regardless, it follows that the resistance between A2 and the SCF isapproximately 0.2531 Ohms and the resistance between B2 and the SCF isapproximately 0.0703 Ohms. In this case, the processing unit 18calculates the RR to be approximately 78%, indicating that the SCF islocated approximately 78% of the CL between A2 and B2.

According to the interconnection information, the TCL between A2 and B2is supposed to be approximately 276 inches, with 196 inches of thatbetween A2 and TB2, 42 inches of that between TB2 and TB3, and 38 inchesof that between TB3 and B2. Applying the RR, the SCF is approximately215 inches from A2 and approximately 19 inches from TB2. Thus, the SCFmust be approximately 23 inches from TB3. In this manner, the processingunit 18 can accurately locate faults even in complex electricalcircuits.

It is important to note that if A2 and C2 or B2 and C1 were used insteadof A2 and B2, in the above example, the processing unit 18 would havebeen able to identify that the SCF was generally located between TB2 andTB3. The processing unit 18 would then select other termination points14 that are connected together on either side of the SCF in order tomore accurately locate the SCF. For example, A2 and C1, B2 and C2, or C1and C2 could have been used instead of A2 and B2 to find the secondlocation of the SCF. Additionally, any of the four combinations, A2 andB2, A2 and C1, B2 and C2, or C1 and C2 can be used to verify the RRcalculated using one of the four combinations. Furthermore, all fourcombinations may be used with the resulting RRs being averaged tocalculate a very precise RR. In any case, the processing unit 18preferably indicates all elements adjacent to the SCF.

As discussed above, in alternative embodiments, the tester 10 may notactually locate or position electrical faults. For example, the tester10 may only find faults. Furthermore, while some form of electricaltesting is preferred, electrical testing is not required. As such, instill other alternative embodiments, the tester 10 may not include anyelectrical testing capability.

The operation of the tester 10 has been described above as primarilyperforming continuity type testing, by testing pairs of the terminationpoints 13. However, the tester 10 may also perform isolation typetesting. For example, the tester 10 may actuate the first relay 34associated with A1 and the remaining second relays 36, such as thoseassociated with A1, A2, B1, B2, C1, and C2. In this case, X1 would beconnected to T1 and the remaining termination points 13 would beconnected to T2. In this manner, the tester 10 can test isolationbetween X1 and the remaining termination points 13.

Fiber Optical Testing

Referring to FIGS. 1 and 7, the fiber unit 22 preferably allows thetester 10 to perform a light attenuation measurement on each of aplurality of fiber strands 42 within the optical circuit 14. Theattenuation measurement is similar to the resistance measurementdescribed above. Each fiber strand 42 is preferably 100/140 micronmulti-mode fiber selected to guide light having an approximately 850nanometer (nm) wavelength. Specifically, the fiber strands 42 preferablyhave approximately 100 micrometer (um) diameter cores surrounded bycladding having an approximately 140 um external diameter. However, thefiber unit 22 may be designed for use with other commonly usedmulti-mode fiber stands, such as 50/125 micron fiber, 62.5/125 micronfiber, and 110/125 micron fiber. Furthermore, the fiber unit 22 may bedesigned for use with commonly used single-mode fiber strands, such as9/125 micron fiber. Finally, the optical circuit 14 may include commonoptical elements, such as couplers, splitters, and switches.

Each fiber strand 42 preferably includes two termination points 15, suchas F1, F2, F3, and F4, with one of those termination points 15 on eitherend of the optical circuit 14. Thus, it can be seen that the opticalcircuit 14 is preferably simpler than the electrical circuit 12described above. For example, each termination point 15 of the opticalcircuit 14 is preferably connected through one fiber strand 42 to oneother termination point 15. Conversely, as discussed above, theelectrical circuit 12 is expected to include termination points 13 thatmay be connected to two or more other termination points 13. Thus, theinterconnection information associated with the optical circuit 14 isalso expected to be simpler than that associated with the electricalcircuit 12. For example, the interconnection information associated withthe optical circuit 14 is expected to include information as to whichpairs of termination points 15 are expected to be interconnected. Theinterconnection information associated with the optical circuit 14 mayalso include information as to sizes, types, and lengths of the fiberstrands 42.

Referring also to FIG. 8, the fiber unit 22 is designed to individuallytest each fiber strand 42. Therefore, in a preferred embodiment, thefiber unit 22 comprises a plurality of fiber optic light ports 44, withat least one light port for each termination point of the opticalcircuit. For example, if the optical circuit 14 comprises twelve fiberstrands each having two termination points 15, the fiber unit 22preferably comprises at least twenty-four light ports 44. Each lightport 44 preferably comprises a light transmitter 46 to transmit lightinto one of the fiber strands 42 through an optical coupler 48, a lightmonitor 50 to monitor the light from the light transmitter 46 throughthe coupler 48, and a light receiver 52 to receive light from the lighttransmitter 46 through one of the fiber strands 42. Referring also toFIG. 9, the light transmitter 46 preferably comprises a light emittingdiode (LED) 54 powered by a constant current source 56. Morespecifically, the LED 54 preferably transmits light at a knowntransmitted power level (PT) with an approximately 850 nm wavelength.

The coupler 48 preferably divides the light transmitted by the lighttransmitter 46 into two equal halves, such as a fiber power (PF) and amonitor power (PM). Thus, the coupler 48 is preferably a 50/50 coupler.More specifically, the coupler 48 preferably directs substantially halfof the light, PF, to one of the termination points 15 of the opticalcircuit 14 and substantially half of the light, PM, to the light monitor50. It can be seen that PF added to PM substantially equals PT and thatPF preferably substantially equals PM. The coupler 48 also preferablydivides the light received from one of the fiber strands 42 into twoequal haves, such as a receiver power (PR) and a waste power (PW). Itshould be noted the PW is preferably directed toward the transmitter 46and has virtually no significance.

Alternatively, the coupler 48 may divide the light transmitted by thelight transmitter 46 into another ratio. For example, the coupler 48 maybe a 67/33 coupler. In this case, the coupler 48 would directapproximately 67% or two thirds of the light to one of the terminationpoints 15 of the optical circuit 14 and approximately 33% or one thirdof the light to the light monitor 50. It should be obvious that otherratios may be used.

It can be seen that the coupler 48 allows the tester 10 of the presentinvention to transmit and receive light with each light port 44 througheach fiber strand 42. Therefore, using the coupler 48, the tester 10 maytest each fiber strand 42 in both directions, without any form of fiberswitching or reversing of connectors. In fact, all fiber components arepreferably solid-state resulting in a durable and rugged tester that canbe moved around in a production environment and still be relied upon tomake accurate, stable, and repeatable attenuation measurements.

However, in an alternative embodiment, the fiber unit 22 may incorporateas few as two light ports 44. In this case, the fiber unit 22 preferablyincludes an optical switch similar in function to the switching unit 20described above. The optical switch facilitates connecting the lightports 44 to each of the termination points 15 of the optical circuit 14.The optical switch preferably comprises a single pole multi-throwoptical relay connected to each coupler 48. Each optical relay ispreferably connected to one of the couplers 48 such that each coupler 48may be connected to any one the termination points 15 of the opticalcircuit 14. Thus, the optical switch may optically connect each fiberstrand 42 between two light ports 44.

The light monitor 50 preferably comprises a monitor photo diode 58 and amonitor operational amplifier 60 that operate together to generate amonitor signal in response to PM. The monitor diode 58 preferably altersinternal electrical characteristics in response to PM and the monitoroperational amplifier 60 preferably detects such altered characteristicsto produce the monitor signal representative of PM. The light monitor 50preferably also includes a monitor logarithmic amplifier 62 thatamplifies the monitor signal to produce a monitor voltage (VM), which ispreferably logarithmically proportional to PM, or the light receivedfrom the light transmitter 46 through the coupler 48.

The light receiver 52 is preferably similar to the light monitor 50 andcomprises a receiver photo diode 66 and a receiver operational amplifier68 that operate together to generate a receiver signal in response toPR. The receiver diode 66 preferably alters internal electricalcharacteristics in response to PR and the receiver operational amplifier68 preferably detects such altered characteristics to produce thereceiver signal representative of PR. The light receiver 52 preferablyalso includes a receiver logarithmic amplifier 70 that amplifies thereceiver signal to produce a receiver voltage (VR), which is preferablylogarithmically proportional to PR, or the light received from the fiberstrand 42 through the coupler 48.

As discussed above, two light ports 44 are preferably used to test eachfiber strand 42. For example, the light transmitter 46 of a first lightport is preferably used to transmit the light into one of the fiberstrands 42, with the light monitor 50 of the first light port monitoringthat light transmitter 46. At the same time, the light receiver 52 of asecond light port is preferably used to receive light from that fiberstrand 42. Thus, the light monitor 50 detects how much light istransmitted into that fiber strand 42 while the light receiver 52detects how much light is received from that fiber strand 42.

The light monitor 50 and the light receiver 52 are preferably connectedto a monitor differential amplifier 74 and a receiver differentialamplifier 76, respectively. The differential amplifiers 74, 76preferably amplify VM and VR with respect to a reference ground bus. Thedifferential amplifiers 74, 76 are preferably connected to an analog busswitch 78 that provides functionality similar to the switching unit 20described above. The analog bus switch 78 preferably connects either themonitor differential amplifier 74 or the receiver differential amplifier76 to one of the test points 27 of the testing unit 16 according toinstructions received by the processing unit 18. For example, with T1connected to one of the monitor differential amplifiers 74 and T2connected to one of the receiver differential amplifiers 76 through theanalog bus switch 78, the voltage sensor 30 can take the voltagemeasurement between VM and VR for one of the fiber strands 42.

It should be obvious that PR is substantially half of PF minus anyattenuation loss in that fiber strand 42. Since PM substantially equalsPF, PR is expected to be substantially half of PM minus the attenuationloss in that fiber strand 42. As discussed above, VM is preferablyproportional to PM and VR is preferably proportional to PR.Additionally, the receiver amplifier preferably amplifies VR such thatVR is expected to be substantially equal to VM minus the attenuationloss in that fiber strand 42. Thus, the voltage measurement between VMand VR is indicative of the attenuation loss in that fiber strand.

The processing unit 18 preferably converts the voltage measurement intoa power loss expressed in decibels (dB). For example, if the fiber unit22 is designed to measure 0 dB to 10 dB, then the processing unit 18 maybe programmed to equate an approximately 50 millivolt (mV) voltagemeasurement with 1 dB. The processing unit 18 preferably indicates theattenuation measurement for each fiber strand 42. Additionally, if anattenuation measurement exceeds a programmed attenuation limit, theprocessing unit 18 also preferably indicates that the associated fiberstrand 42 has failed. For example, the tester 10 may take attenuationmeasurements of a specific fiber strand in first and second directions.If the technician has specified approximately 4 dB as the attenuationlimit and the attenuation measurement is approximately 3.8 dB in thefirst direction and approximately 4.1 dB in the second direction, thenthe processing unit 18 preferably indicates both attenuationmeasurements and that the specific fiber strand has failed.

As discussed above, each light port 44 preferably directs PF toward oneof the termination points 15 and receives PR from one of the terminationpoints 15 of the optical circuit 14. While the light ports 44 may beconnected directly to the termination points 15, the light ports 44 arepreferably enclosed within a protective body of the tester 10, and thusnot accessible without removing an access panel 80 of the body.Therefore, the termination points 15 of the optical circuit 14 arepreferably connected to the access panel 80 using interface connectors82. The interface connectors 82 are preferably selected specifically tomate with the termination points 15 of the optical circuit 14 and may beany combination of commonly used connectors, such as FC, FDDI, LC, MTArray, SC, SC Duplex, and ST. Alternatively, the optical harness maymate with the interface connectors 82 and include specially adaptedconnectors to mate with the optical circuit 14.

Additionally, the couplers 48 of the light ports 44 are preferablyconnected to the interface connectors 82 through patch cords with highquality ST connectors. The patch cords are preferably constructed from agraded-index fiber that has a substantially parabolic refractive indexprofile and an approximately 0.29 numerical aperture (NA). The parabolicrefractive index functions to redirect the light back toward a center ofthe graded-index fiber's core, in an effort to reduce multi-modedistortion. Similarly, the light transmitters 46, the light monitors 50,and the light receivers 52 are preferably individually connected to thecouplers 48 through step-index fibers that have an approximately 0.29NA.

The fiber unit 22 is preferably designed to use narrowly filled launchcharacteristics (NFLC) that focus the light toward the center of thefiber strands according to a launch condition standard, such as anAS-100 specification which is currently a proposed specification. NFLCminimizes the effects of variations and imperfections in the connectorsand provides more consistent and repeatable attenuation measurements.However, the fiber unit 22 may be designed to use fully filled launchcharacteristics (FFLC) that more evenly distributes the light across theNA of the fiber strands, which tends to accentuate the effects ofvariations and imperfections in the connectors. Additionally, the fiberunit 22 may be designed to use other launch characteristics, asspecified for a particular application.

In order to accomplish the launch characteristics, each light port 44may include a launch filter 84. While the filter 84 is preferablyconnected between the coupler 48 and the interface connector 82, thefilter 84 may be connected between the light transmitter 46 and thecoupler 48. The filter 84 preferably comprises a graded-index fiberfused to a step-index fiber combined with a mode scrambler. The modescrambler is preferably a set of equally spaced pins through which afilter fiber is woven producing uniform and opposing bends in the filterfiber. The pins are set to a particular spacing derived from the fiberstrand's size to provide appropriate micro-bending, and thus uniformpower distribution. However, the mode scrambler may be any other devicethat provides a specified power distribution.

As discussed above, the tare log is developed during calibration of thetester 10 and stores internal characteristics of the tester 10. Theinternal characteristics may include attenuation characteristics of thefiber unit 22. Thus, the processing unit 18 can compensate for internalcharacteristics of the fiber unit 22, as well as other internalinfluences of the tester 10, as described above.

Furthermore, the tare log can store interface characteristics. Forexample, if the fiber unit 22 is used with a differently sized fiberstrands, such as 62.5/125 micron fiber, the interface connectors 82 mayinduce attenuation loss and drastically influence the attenuationmeasurements. Such influences may lead the processing unit 18 toerroneously determining that the fiber strand 42 failed, since theattenuation measurement is likely to exceed the attenuation limit.However, the fiber unit 22 can be calibrated with the differently sizedfiber strands, thereby including the attenuation loss at the interfaceconnectors 82 in the tare log. Thus, while the processing unit 18compensates for the internal characteristic, the processing unit 18 alsocompensates for the attenuation loss at the interface connectors 82.

SUMMATION

As discussed above, the processing unit 18 indicates faults, faultlocations, fault positions, and whether each portion of the circuits 12,14 pass or fail tests performed by the tester 10. It is important tonote that these indications are preferably given in two forms. First,the processing unit 18 preferably indicates these results on anelectronic display, such as a computer monitor. Second, the processingunit 18 preferably generates a report including text and/or graphics,which may be dumped to a file or printed through a printer.

The processing unit 18 of the tester 10 is preferably controlled andmonitored through a user interface program. The program preferablyallows the technician to view and print the report detailing themeasurements taken from the circuits 12, 14. Additionally, the programmay allow the technician to provide the interconnection information.Furthermore, the program may allow the technician to specify the mannerin which the processing unit 18 tests the circuits 12, 14.

While the tester 10 of the present invention may be entirely housed withthe body, some components, such as the switching unit 20 may be locatedoutside the body. Additionally, each component of the tester 10 may haveits own protective body. For example, the processing unit 18 may behoused in a first protective body and the fiber unit 22 or a portion ofthe switching unit 20 may be housed in a second protective body. In thismanner, individual components of the tester 10 may be located atsubstantial distances from other components and may communicate overcabling.

As discussed above, the tester 10 may be directly connected to thecircuits 12, 14 or may be connected to the circuits 12, 14 through theelectrical harness 24 and the optical harness. Additionally, the tester10 may be connected to the circuits 12, 14 through other adapter forms,such as adapter plates and custom designed connectors.

While the present invention has been described above, it is understoodthat substitutions can be made. For example, the electrical circuit 12may actually be two or more simple conductors. Additionally, the opticalcircuit 14 may actually be one or more fiber strands. While suchcircuits 12, 14 may not normally warrant such an advanced tester,certain characteristic, such as being extremely long, being buried, orotherwise difficult to access, may require use of the tester 10.Furthermore, the switching unit 20 may incorporate single pole doublethrow electromechanical switches instead of the pairs of relays 32.These and other minor modifications are within the scope of the presentinvention.

Having thus described a preferred embodiment of the invention, what isclaimed as new and desired to be protected by Letters Patent includesthe following:

1. A fiber optic tester operable to test an optical circuit having aplurality of termination points, the tester comprising: a plurality oflight ports operable to transmit and receive light, each light portcomprising a light transmitter operable to transmit the light, a lightreceiver operable to receive and convert the light into a receivervoltage, a light monitor operable to receive and convert the light intoa monitor voltage, and a coupler operable to split the light transmittedfrom the light transmitter between the light monitor and one of thetermination points; a testing unit operable to measure a differentialvoltage between the receiver voltage and the monitor voltage; aprocessing unit operable to analyze differences between the receivervoltage and the monitor voltage; and an analog bus switch operable toconnect each light receiver and each light monitor to the testing unitin a sequence controlled by the processing unit.
 2. The tester as setforth in claim 1, wherein each light port further includes a receiveramplifier connected between the light receiver and the analog bus switchand operable to amplify the receiver voltage.
 3. The tester as set forthin claim 1, wherein each light port further includes a monitor amplifierconnected between the light monitor and the analog bus switch andoperable to amplify the monitor voltage.
 4. The tester as set forth inclaim 1, wherein each light port further includes a receiver amplifieroperable to amplify the receiver voltage, a monitor amplifier operableto amplify the monitor voltage, and an analog bus switch operable toconnect each receiver amplifier and each monitor amplifier to thetesting unit in a sequence controlled by the processing unit.